Watershed- and plot-scale tests of the mobile anion concept

Anion fluxes from a forest soil are usually correlated with those of base c ations (BC). Declines in base cation deposition or long-term depletion from the soil may change these relationships. We used multiple regression to id entify biogeochemical variables predicting annual volume-weighted concentra tions of BC in streamwater draining a forested watershed, and analysis of v ariance to compare the effects of Ca and Cl inputs on BC fluxes out of soil horizons in irrigated plots. For the watershed, anion concentrations in st reamwater predicted BC export most precisely (R-2 = 0.84). The best two-var iable model (adjusted R-2 = 0.91) also included BC concentration in bulk de position. Consistent with predictions from equations governing exchange che mistry, the proportion of charge contributed by Ca2+ increased with increas ing total anion concentration, while that of Na+ decreased. At the plot sca le, Cl- concentrations in treatment solutions had a stronger effect (p = 0. 06) on BC concentration in Oa-horizon solutions than did Ca2+ concentration s (p = 0.33). In individual horizons of individual plots, BC and total ion concentrations were correlated, but cation composition was not consistent w ithin horizons from different plots. This study detected no evidence of lon g-term cation depletion in the soils controlling streamwater, but did detec t extremely base-poor plots. Because acid deposition affects surface horizo ns first, streamwater chemistry may not be an adequate way to assess nutrie nt supply of forest soils.